The effect of prolonged glutamate (GLU) application was examined on 60 CA1 pyramidal neurons in the in vitro rat hippocampal slice preparation. Continuous application of L-GLU, either by bath perfusion (0.5-2 mM) of the slices or iontophoresis (200 mM) into the dendritic region of the neurons, elicited a transient depolarization which faded to a mean of 53% of the initial peak amplitude despite continued exposure to the agonist. Membrane depolarization to aspartate (ASP) and the d-isomer of GLU also faded with time. In contrast, the depolarizing response to the excitatory amino acid agonists N-methyl-D,L-aspartate (NMA), quisqualate (QUIS), and kainate (KA) did not fade significantly during continuous application. The fade of the GLU depolarization was not affected by the NMDA antagonist D-2-amino-5-phosphonovalerate (APV) or by blocking synaptic transmission with tetrodotoxin. At the time of maximum fade of the GLU depolarization, there was no change in input resistance or GLU reversal potential. In addition, fade of the response was not a consequence of changes in extracellular potassium concentration, GLU uptake mechanisms, or the electrogenic pump. The most likely explanation for fade is postsynaptic receptor desensitization.